It is well known in the art to envelop solid pharmaceutical materials, tablets, granules and seeds, for example, in a film covering as protection against oxidation, moisture, light, abrasion, rough handling, etc. The film should be free of roughness, irregularities, cracks or mottled colorations. Film smoothness is important as an aid in swallowing. A hard, shiny surface is desirable for an attractive appearance. Of course, films and coating compositions for ingestion must be edible or physiologically compatible.
Film-forming compositions for coating pharmaceutical tablets preferably contain as the film-forming element, a film-forming resinous material, either naturally occurring or synthetic. Normally, such compositions are applied as a liquid coating formulation comprising a liquid carrier medium having dispersed or dissolved therein the film-forming components. The liquid medium can be an organic solvent or water or a combination of both. Water is preferred owing to the risk of fire and toxicity from organic solvents. Also having to comply with governmental safety standards pertaining to the transportation and handling of industrial chemicals is another minus factor against solvent use.
Generally speaking, application of the liquid coating formulation is effected by spraying dry pharmaceutical forms in rotation in a coating pan or in a fluidized air bed. After evaporation of the liquid medium, the film coated pharmaceuticals are recovered.
As a commercial product, liquid coating compositions are unsatisfactory because of the high transportation costs due to the weight of the liquid carrier. Clearly, it is more practical and economical to ship coating compositions in dry form which can be reconstituted with the appropriate solvent or liquid by the pharmaceutical customer. For this, a readily dispersible product is desirable which can be reconstituted by simple mixing or stirring.
It is known in the prior art to prepare dry coating formulations by grinding a polymer powder with pigment particles and grinding the mixture further to give a fine powder. However, when this is stirred into water, the polymer tends to agglomerate resulting in a nonuniform dispersion which, unless permitted to solvate for an extended period of time, gives poor coatings due to the presence of lumps and fish eyes.
An improved coating system is described in U.S. Pat. Nos. 4,543,370 and 4,683,256 assigned to the Colorcon Corporation. According to these documents, a pharmaceutical coating composition is produced by high intensity blending of polymer and pigment particles in the presence of a plasticizer. The resulting powder is mixed with water to form a coating dispersion which is applied to tablets. On drying, a uniform film is said to be produced on the tablets. The coating can also be applied as a solution in an organic solvent.
The dry coating compositions of the Colorcon patents consist of separate individual particles of pigment and polymer. Although the vigorous dry blending produces a fine powder, its heterogeneous nature is readily discernible under the scanning electron microscope.